Yes the context of what zero emissions means seems simple enough, but there will be those who'll cherry pick right the way though from manufacture and environmental burden.

As for fires, presently not much of an issue bar the immediate issue of a burning car, but presents a problem should the batteries remain as is when there is a time when most cars on the road and large car parks are EV - it would be better if the industry adopted better batteries sooner rather than later rather than count on further income generated when the owners eventually move to a better battery configuration. eg [1]

Toyota has a 6 stroke, sorry I missed that. I follow such engine improvements with interest, but not so much since a 8 stroke was developed and news of it buried. (It was a test bench only engine, and didn't need additional cooling, said to be 225% than standard engine) Sadly I'm unable to find any additional information on it or the Toyota 6 stroke, but then again when I went decided to also look for details on the large 6 strokes that were used for power generation, one such large engine I'm sure was mentioned at length at wikipedia - it too seems to be yet another entry, that's been cleaned up and mention of it dropped. [2]

[1] https://hub.jhu.edu/2019/10/17/lithium-ion-battery-safety/

[2] https://en.wikipedia.org/wiki/Six-stroke_engine